As is conventionally known, in a diesel engine used as an automobile engine or the like, fuel injection control is performed that adjusts a fuel injection timing and a fuel injection amount from a fuel injection valve (also referred to below as an injector) according to an operating state, such as the engine revolutions, amount of accelerator operation, coolant temperature, and intake air temperature.
Incidentally, diesel engine combustion is composed of premixed combustion and diffusive combustion. When fuel injection from a fuel injection valve begins, first a combustible mixture is produced by vaporization and diffusion of fuel (ignition delay period). Next, this combustible mixture self-ignites at about the same time at numerous places in a combustion chamber, and combustion rapidly progresses (premixed combustion). Further, fuel injection into the combustion chamber is continued, so that combustion is continuously performed (diffusive combustion). Afterward, unburned fuel exists even after fuel injection has ended, so heat continues to be generated for some period of time (afterburning period).
Also, in a diesel engine, as the ignition delay period grows longer, or as the vaporization of fuel during the ignition delay period grows more intense, a flame propagation speed after ignition will increase. When this flame propagation speed is large, the amount of fuel that burns at once becomes too great, pressure inside the cylinder drastically increases, and so vibration or noise occurs. Such a phenomenon is called diesel knocking, and often occurs particularly when operating with a low load. Also, in this sort of situation, a drastic elevation in burn temperature is accompanied by an increase in the amount of nitrogen oxide (referred to below as “NOx”) produced, and thus exhaust emissions become worse.
Consequently, in order to prevent such diesel knocking and reduce the amount of NOx produced, various fuel injection control apparatuses have been developed. For example, a fuel injection control apparatus has been developed whereby pilot injection that injects a small amount of fuel is performed prior to main injection that causes combustion contributing to the production of engine torque. That is, there have been proposals in which, after executing pilot injection, fuel injection is temporarily interrupted, and when that fuel has reached an ignition state (a so-called flash point), main injection is executed (see below Patent Citations 1 to 3).
By executing this sort of pilot injection, it is possible to mitigate the initial combustion that accompanies the start of subsequent main injection, and thus it is possible to suppress the occurrence of diesel knocking. Also, the fuel injected in pilot injection is already ignited when executing main injection, and the flash point state has already been established, so it is also possible to avoid the occurrence of misfire. Therefore, with pilot injection, low temperature starting is improved, and the occurrence of white smoke at low temperature also is reduced. Furthermore, with this pilot injection, the amount of fuel injection during the ignition delay period is reduced, so premixed combustion also is suppressed. Also, during the premixed combustion, the heat generation rate is high, so it is possible that generation of NOx will be promoted, but because the premixed combustion is suppressed with the above pilot injection, the production of NOx and the production of noise that accompanies the premixed combustion are both likewise reduced.
However, in order to perform pilot injection of a small amount of fuel, it is necessary to control the amount of this pilot injection with high precision. For example, in Patent Citation 2, pilot injection is executed such that a compression end pressure, which is the maximum pressure that occurs within the cylinder in the engine compression stroke, approaches a target compression end pressure. In this case, a smaller amount of pilot injection is set as the difference between the target compression end pressure and the actual compression end pressure grows smaller.
Also, Patent Citation 3 discloses increasing the amount of pilot injection as the intake air temperature decreases, and as the intake air pressure increases.
[Patent Citation 1]
JP 2004-308440A
[Patent Citation 2]
JP 2000-154757A
[Patent Citation 3]
JP 2006-274997A